I would spend 2-3 days at Gettysburg. I would take a day to see Harper's Ferry and Antietam. All three sites are beautiful.
Now at Gettysburg, I would invest in a Gettysburg battlefield guide for as many hours as you can afford. You will be rewarded by an excellent narrative. On Gettysburg, I recommend a good guidebook. I really like the Complete Gettysburg Guide by J.D. Petruzzi. That book has maps on the battle itself, but it also includes stops on field hospitals, rock carvings, historic buildings, and details on both the National Cemetery and the adjacent Evergreen Cemetery. Read the guidebook BEFORE your visit. That book gives plenty advice on seeing interesting things not on most Gettysburg tours.
Gettysburg Stone sentinels is a website that gives a lot of detail. It is a great resource if you want to find specific monuments, markers and historic buildings.
I have done Harper's Ferry and Antietam in one day several times. You can spend a lot more time at Antietam but seeing the sunken Lane and Burnsides Bridge are the highlights.
Obviously CWT is a valuable resource as well. Now if you think Gettysburg is all you need for a CW battlefield. You have Nearby Philadelphia, Baltimore and D.C. That has world class historic sites for history buffs.